This invention generally relates to apparatus and methods for expanding tubular members and more particularly relates to a system and a method for hydraulically expanding heat transfer tubes and sleeves of the type found in nuclear steam generators.
Although devices and methods for expanding nuclear steam generator tubes and sleeves are known, it has been observed that these devices and methods have a number of operational problems associated with them. However, before these problems can be appreciated, some background is necessary as to the structure and operation of a typical nuclear steam generator.
In this regard, a typical nuclear steam generator generates steam when heat is transferred from a heated and radioactive primary fluid to a non-radioactive secondary fluid of lower temperature. The primary fluid flows through a plurality of U-shaped tubes (i.e., a tube bundle), which are received through holes in a plurality of support plates and holes in a tubesheet, which support plates and tubesheet are disposed in the steam generator. The secondary fluid flows across the exterior surfaces of the tubes as the primary fluid flows through the tubes. The walls of the tubes function as heat conductors for transferring heat from the heated primary fluid flowing through the tubes to the secondary fluid of lower temperature flowing across the exterior surfaces of the tubes.
Because the primary fluid is radioactive, the steam generator is designed such that the radioactive primary fluid flowing through the tubes does not radioactively contaminate the nonradioactive secondary fluid flowing across the exterior surfaces of the tubes. Radioactive contamination of the secondary fluid is undesirable for safety reasons. Therefore, the tubes are designed to be leak-tight so that the radioactive primary fluid remains separated from the nonradioactive secondary fluid to avoid commingling the primary fluid with the secondary fluid.
Occasionally, due to tube wall intergranular cracking caused by stress and corrosion during operation (i.e., known in the art as primary water stress corrosion cracking), the steam generator tubes may degrade (i.e., experience tube wall thinning) and thus may not remain leak-tight. If through-wall cracking occurs due to the degradation, the radioactive primary fluid may commingle with the nonradioactive secondary fluid. However, if degradation is suspected, the tube, although degraded, may remain in service by sleeving the degraded portion of the tube. When sleeving is performed, a tubular sleeve is inserted into the tube to cover the degraded portion of the tube. The sleeve is then secured to the tube by radially expanding portions of the sleeve into intimate engagement with the inner wall of the tube, such that the degraded portion of the tube is spanned or covered. In this manner, the radioactive primary fluid is prevented from commingling with the non-radioactive secondary fluid even though the wall of the tube is degraded.
Moreover, during operation of the steam generator the tubes may experience undesirable large-amplitude vibration due to the energy absorbed by the tubes from impingement of the high-velocity secondary fluid on the exterior surfaces of the tubes, such as may occur in postulated accident conditions (e.g., loss of coolant accident) during operation of the steam generator. Such tubes can become unstable due to these relatively large vibration amplitudes. Unless mitigated, these vibration amplitudes may generate sufficient levels of bending stress to enhance the initiation and propagation of any degradation that may be present in the wall of the tube. The combined vibration and degradation may cause the tube to sever, thereby commingling the primary fluid with the secondary fluid. The severed unsupported tube ends are also then free to impact against and damage adjacent tubes. Thus, if this were to occur, the tube ends may swirl in the flowing fluid field and impact neighboring undamaged tubes in the tube bundle with force sufficient to damage these neighboring tubes in which primary fluid is still flowing. The walls of these neighboring tubes may thereby become breached and allow the radioactive primary fluid to radioactively contaminate the non-radioactive secondary fluid. Therefore, to mitigate such large-amplitude vibration, some or all of the tubes may be radially expanded into engagement with its surrounding support plates or tube sheet in order to stabilize the tube so that the tube will not experience large-amplitude vibration, even during postulated accident conditions.
Both sleeving and stabilization of the tubes may be obtained by radial expansion of the tubes and/or sleeves. That is, such sleeving or stabilization is performed by either expansion of a sleeve into engagement with the inner wall of the tube for sleeving the tube or expansion of the tube itself into engagement with its surrounding support plates or tubesheet to mitigate vibration of the tube.
However, the required length of the expansion zone of the tube and/or sleeve in the axial direction may be relatively short (e.g., less than or equal to approximately 0.6 inches). Such a short axial length is present between the U-bend region of each tube and the top surface of the uppermost support plate. Applicant has discovered that prior art devices have proven unsatisfactory for expanding such short axial lengths. For example, such prior art devices fail after repeated use because the short axial length of the expansion member belonging to such devices lack sufficient strength and resiliency to withstand the relatively greater radial pressure required to expand the tube over a short axial length.
Devices for expanding tubes are known. One such device is disclosed by U.S. Pat. No. 4,724,595 issued Feb. 16, 1988 in the name of David A. Snyder entitled "Bladder Mandrel For Hydraulic Expansions Of Tubes And Sleeves" and assigned to the assignee of the present invention. However, this Snyder patent does not appear to disclose a system and method for suitably expanding relatively short axial sections of tubing.
Another device for expanding tubes is disclosed in U.S. Pat. No. 4,159,564 issued Jul. 3, 1979 in the name of Frank W. Cooper, Jr. entitled "Mandrel For Hydraulically Expanding A Tube Into Engagement With A Tubesheet" and assigned to the assignee of the present invention. However, the Cooper, Jr. patent does not appear to disclose a system and method for suitably expanding relatively short axial sections of tubing.
Hence, although devices and methods for expanding tubes are known in the art, the prior art does not appear to disclose a system and method for hydraulically expanding tubular members, wherein the required axial length of the expansion zone for the tubular member may be relatively short.
Therefore, what is needed is a system and method for hydraulically expanding tubular members, wherein the required axial length of the expansion zone for the tubular member may be relatively short.